1. Field of the Invention
This invention relates to apparatus and a method for image-guided surgery or therapy. More particularly, it relates to apparatus which utilizes A-mode ultrasound to register physical space with a previously generated tomographic image to aid a surgeon in procedures such as brain surgery.
2. Background Information
In image-guided surgery, a previously generated tomographic image such as a computed tomography (CT) scan or a magnetic resonance image (MRI) is used to direct the surgeon to the site of interest. It is particularly useful in neurosurgery especially when a small entry through the skull is made or where the boundaries of the target tissue are not visible to the naked eye but are well delineated in the tomographic image. An important task in such a procedure is to define a relationship between the patient and the previously obtained images of the patient. In order to implement this task, a coordinate system is assigned to the physical space occupied by the patient. Since the images of a patient contain their own coordinate system, the task of relating the images to the patient can be accomplished by determining a mathematical relationship between the image coordinate system and the patient coordinate system. This mathematical relationship between coordinate systems is referred to as a registration and the two coordinate systems are said to be registered with respect to each other. When the two coordinate systems both arise from images of the patient, the registration is referred to as an image-space to image-space registration, or more simply an image-to-image registration. When one of the coordinate systems represents the actual patient, the registration is classified as an image space to physical space registration. Ideally, a point defined in one coordinate system will map to the identical point defined in the other coordinate system, but in reality, there is always some error associated with the relationship.
A known image-guided procedure is stereotactic surgery. In this technique, a large metal frame is secure by pins imbedded in the skull. An image such as a CT image of the head is then taken with the frame in place. A computer system is used to calculate settings on arcs on the frame for positioning instruments such as a biopsy probe to take a sample at a desired location in the brain. This is an invasive procedure which requires implantation of the pins in the skull. Images made before attachment of the pins in the frame are not useable. The stereotactic technique is not suitable for serial treatments as there are limitations on the duration that the pins can remain in place. Also, the pins project out of the skull and can be bent or misaligned. While the frame is also used to immobilize the patient's head, it is heavy and uncomfortable.
There have been efforts to eliminate the heavy frame required in stereotactic procedures. Most of the technologies involve registering skin surface in physical space and image space. Various systems have been proposed for establishing coordinates in physical space by tracking a probe as it is scanned over the skin surface. These include: articulated arm, active and passive optical, ultrasonic, electromagnetic and inertial systems. Such approaches have not resulted in very good accuracy however, mainly because the skin is deformable and thus establishing the precise position of the probe relative to undisturbed skin surface is difficult.
A different proposal suggests the implantation in the skull of fiducial markers which will appear in a subsequently taken tomographic image. An A-mode ultrasound transducer is then scanned over the skull to detect the position of the fiducial markers in physical or patient space. An optical tracking systems determines the position of the transducer in physical space. An algorithm is used to correlate the position of the probe in physical space to corresponding points on the skull in the image. Once this correlation has been made, the transducer can be replaced by a surgical probe which is also tracked by the optical tracking system. The position of the probe is projected onto the image to guide the surgeon in aligning entry through the skull. This procedure does not require any frame as in the stereotactic procedure. Furthermore, the fiducial markers can be flush with the surface of the skull. However, it is still an invasive procedure requiring implants in the skull.
A-mode ultrasound is used in dermatology, radiation oncology and even by farmers, all to measure skin thickness, but not to map bone surfaces.
There is a need for an improved non-invasive system and method for image-guided surgery and therapy.
In particular, there is a need for such a system and method which does not require implants in the skull.
There is an additional need for such a system and method which can utilize previously generated tomographic images thereby eliminating the need to take additional images at the time of surgery.
There is also a need for such a system and method which can be used for a sequence of treatments over an extended period of time.
There is a need for a system and method that meets all of these needs and yet is accurate and robust.